Such a device is disclosed in British Patent Specification No. GB-A-2139169, and comprises a stationary blade projecting radially from a ring and restrained against rotation by cooperation with a propeller shaft bearing, and a pair of diametrally opposed rotatable blades projecting radially from another ring to be secured to the propeller shaft. The blades have straight cutting edges on both sides lying radially to the rings, so that the width of the blades increases radially outwardly to form fan-shaped or dovetailed blades. At the distal or radially outward free end of each blade integral ear portions project forwardly and rearwardly, in the direction of rotation. Thus the ear portion on the leading edge of a rotating blade engages the ear portion on the opposed edge of the stationary blade before the cutting edges co-operate to shear a line therebetween. The intention of the ear portions is to prevent jamming of the blades as they wear. As the cutting edges approach during rotation, a line caught in the space therebetween tends to slide radially outwardly along the cutting edges until stopped by the ear portions, so that cutting usually takes place with the line at the distal or radially outward ends of the cutting edges. The moment of the cutting forces when cutting takes place there is greater than if it takes place closer to the rings. In addition, cutting takes place through the full width of a line in one action and this may prove difficult or impossible with large lines.